“What we have uncovered appears to be intentional doping of the worst sort,” said USADA chief Terry Madden.

In what is described as the largest drug bust in athletics history, the USADA said a number of international athletes had also tested positive and there had also been positive samples for the stimulant modafinil.

The substance tetrahydrogestrinone was initially found during analysis of the contents of a syringe handed in, under anonymity, by a “top” athletics coach.

“This is a conspiracy involving chemists, coaches and certain athletes using what they developed to be undetectable designer steroids,” said Madden.

“This is a very sophisticated designer steroid created by very sophisticated chemists.

The International Association of Athletics Federations and the involved athletes have been notified of the findings by USADA.